Restructure nu-protocol in more meaningful units (#11917)

This is partially "feng-shui programming" of moving things to new
separate places.

The later commits include "`git blame` tollbooths" by moving out chunks
of code into new files, which requires an extra step to track things
with `git blame`. We can negiotiate if you want to keep particular
things in their original place.

If egregious I tried to add a bit of documentation. If I see something
that is unused/unnecessarily `pub` I will try to remove that.


- Move `nu_protocol::Exportable` to `nu-parser`
- Guess doccomment for `Exportable`
- Move `Unit` enum from `value` to `AST`
- Move engine state `Variable` def into its folder
- Move error-related files in `nu-protocol` subdir
- Move `pipeline_data` module into its own folder
- Move `stream.rs` over into the `pipeline_data` mod
- Move `PipelineMetadata` into its own file
- Doccomment `PipelineMetadata`
- Remove unused `is_leap_year` in `value/mod`
- Note about criminal `type_compatible` helper
- Move duration fmting into new `value/duration.rs`
- Move filesize fmting logic to new `value/filesize`
- Split reexports from standard imports in `value/mod`
- Doccomment trait `CustomValue`
- Polish doccomments and intradoc links

crates/nu-parser/src/exportable.rs

@@ -0,0 +1,8 @@
+use nu_protocol::{DeclId, ModuleId, VarId};
+
+/// Symbol that can be exported with its associated name and ID
+pub enum Exportable {
+    Decl { name: Vec<u8>, id: DeclId },
+    Module { name: Vec<u8>, id: ModuleId },
+    VarDecl { name: Vec<u8>, id: VarId },
+}

crates/nu-parser/src/lib.rs

@@ -1,4 +1,5 @@
 mod deparse;
+mod exportable;
 mod flatten;
 mod known_external;
 mod lex;

crates/nu-parser/src/parse_keywords.rs

@@ -1,4 +1,5 @@
 use crate::{
+    exportable::Exportable,
     parse_block,
     parser_path::ParserPath,
     type_check::{check_block_input_output, type_compatible},
@@ -13,7 +14,7 @@ use nu_protocol::{
     },
     engine::{StateWorkingSet, DEFAULT_OVERLAY_NAME},
     eval_const::eval_constant,
-    span, Alias, BlockId, DeclId, Exportable, Module, ModuleId, ParseError, PositionalArg,
+    span, Alias, BlockId, DeclId, Module, ModuleId, ParseError, PositionalArg,
     ResolvedImportPattern, Span, Spanned, SyntaxShape, Type, Value, VarId,
 };
 use std::collections::{HashMap, HashSet};